doc: Add additional run-time licensing info for Android

Add the documentation detailing the license of the libstdc++
library which is a dependency of Qt apps on Android.

Task-number: QTBUG-31241
Change-Id: I90f3eea86cc89aba1b80bee9c249caee8d223cfa
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>